This invention relates to a method of producing various clad steel articles.
Steel materials such as steel tubes, steel plates, and steel shapes are being used widely and in great quantities in an extensive range of industrial fields for structures such as bridges and buildings, of course, and also for oil-well pipes, various piping and pipe-lines, heat exchangers, and many other items of equipment. These steel materials are required to have not only mechanical strength sufficient for their uses but also properties such as heat resistance, corrosion resistance, and abrasive wear resistance, which are important for durability of the steel materials.
While the physical chemical material conditions of these steel materials with respect to their material properties, thickness, shape, etc., are determined from the strength requirement as a first consideration, the conditions for the requirement for the above mentioned durability do not coincide with those for strength in many instances. For example, a steel of high tensile strength does not necessarily have excellent corrosion resistance.
Accordingly, it has been the practice to resort to measures such as cladding, simple lining, and coating with appropriate metal materials, single non-metallic materials, mixtures thereof, and the like for protecting the steel materials.
Particularly in recent years there is a trend toward the building of larger and more complicated structures such as plants. This trend has given rise to great increases in not only initial construction costs but also in running cost relating to operation and maintenance for such measures as corrosion proofing, whereby there is an ever increasing need for steel materials having ample corrosion resistance together with strength.
For example, cases wherein steels of high tensile strength are used as reinforcing materials are increasing. However, high-tensile-strength steels, contrary to their excellent strength characteristic, are not necessarily fully satisfactory on the point of corrosion resistance. Accordingly, as a countermeasure, it is possible to use a stainless steel which has excellent corrosion resistance. A stainless steel, however, is somewhat inferior in strength, whereby it has the demerit of giving rise to thick and heavy construction and high costs.
Accordingly, there has been proposed a technique wherein, with the aim of utilizing the advantages of the two steels thereby to make up for their deficiencies, a high-tensile-strength steel is used as the base metal, and a stainless steel is caused to adhere intimately to the base metal thereby to impart their respective strength and corrosion resistance characteristics to the combination. In this case, it is highly desirable on the points of economy and strength of materials that the expensive bonded stainless steel be thin and that the strength of the bond between the metals be high.
As a technique for bonding intimately to one metal material a metal of a different kind, the cladding method is commonly used. For carrying out this cladding, there are, for example, the explosive-cladding or percussion-cladding method, the weld-overlaying method, and the hot-rolling method.
The explosive-cladding method utilizes the application of instantaneous high pressure accompanying the explosion of an explosive. This method requires expensive equipment including safety measures and means, whereby it entails high initial costs, high running costs, and complicated operation and is not easily practiced.
The weld-overlaying method comprises welding and adding a molten padding of a stainless-steel filler material on the surface of a base material of carbon steel. This method is accompanied by the problems such as welding strain imparted by the welding heat during the welding, the long time required for the welding work, low work efficiency, and limitation of the selectable filler materials of good weldability.
The method of producing electric-resistance welded steel pipes from clad steel strip formed into tubes, which is similar to this, also requires much labor and has other drawbacks such as precipitation of carbides due to heat of welding and the necessity of homogenizing the welded structure by heat treatment after welding.
The hot-rolling method is a historically old technique in which a metal material is placed intimately against a metal material of another kind after their contact faces have been cleansed, and then the two materials are hot rolled, in which process step, fresh surfaces are generated on the contact faces and the two materials are roll welded while they are still at a high temperature. Accordingly, the provision beforehand of clean contact faces is an indispensable condition. Therefore, in order to satisfy this condition, the presence of residual air in the contact faces and the formation of oxide films thereon must be absolutely avoided, whereby there are cases wherein a measure such as preserving the contact faces in a vacuum becomes necessary. Furthermore, this method requires a large reduction ratio and a high rolling temperature, for which various measures have heretofore been resorted to, but the means have been disadvantageously complicated and inefficient in many instances.
For the above mentioned bonding, the friction welding method, per se, is a historically known method. In this method, there is almost no generation of heat from parts other than the faces being bonded, and plastic flow occurs at the bond faces. Accordingly, not only is the energy efficiency high, but there are other excellent merits such as the attainment of maximum bond strength through solid-state bonding.
Furthermore, this bonding method affords a range of selection of combinations of bondable metals of different kinds which is very much broader than the range afforded by other bonding methods. Moreover, because of the solid-state bonding based on plastic flow as mentioned above, no metal compounds are formed. Further, there is little welding deformation, and high dimensional precision is afforded, whereby this method can be said to be highly suitable for clad welding.
Additional merits of this method are that welding rods and a welding flux are not required, and that a pretreatment of the faying faces to be bonded is unnecessary, whereby economy in materials, decrease in labor, short welding time, high work efficiency automation of the bonding process, high reliability, and other advantageous features are afforded. It is evident, therefore, that this friction welding method is optimally suited for the production of clad steel materials.